Battery packs disclosed in Patent Documents 1 and 2 are power sources for use in portable information technology devices, such as a cellular phone and a mobile computer. The battery back has such a structure that a power generating element such as lithium ion is accommodated in an outer package made of an aluminum laminated film (a laminated sheet), constituting a secondary battery, and the secondary battery is housed in a plastic outer case (pack case). This configuration is to prevent the outward expansion or the change of outline of the outer case (pack case) when the outer package expands due to the increased internal pressure of the secondary battery.
In Patent Document 1, the flat outer case made of synthetic resin has openings on both sides of the case in the direction of thickness and metal plates are stuck on the sides to cover the openings respective so that the outward expansion of the battery body is released inside the openings.
In Patent Document 2, a pack case housing a secondary battery is provided with space formation parts between the secondary battery and the case, forming spaces between the inner wall of the case and the secondary battery. When the outer package made of a laminated sheet expands due to the generation of gas, the expansion of the outer package is released inside the space formation part, thereby preventing the change in the outline of the pack case.
A secondary battery disclosed in Patent Document 3 is a lithium secondary battery to be used as a battery for portable electronics or personal computers. This battery is adapted to prevent breakage of a battery element covered with a flexible synthetic-resin film.
In the secondary battery disclosed in Patent Document 3, a rigid retainer is placed in contact with the sides of a battery element, and the outer faces of the battery element are covered with the flexible synthetic-resin film. This is to prevent a load on the battery element during a process of covering the battery element with the synthetic-resin film or during use, and thus prevent damages or short circuits in the battery element.
The above mentioned prior art documents are as listed below.    Patent Document 1: Jpn. unexamined patent publication No. 2001-196035 (paragraphs [0002]-[0008])    Patent Document 2: Jpn. unexamined patent publication No. 2001-57190 (paragraphs [0002]-[0008])    Patent Document 3: Jpn. unexamined patent publication No. 2000-195475 (paragraphs [0002]-[0008], [0041])
Patent Documents 1 and 2 mentioned above relate to the battery pack comprising the secondary battery in which the battery element is accommodated in the outer package such as a laminated film and further housed in the pack case for use. This battery pack, in which the pack case has the openings or the space formation part, may absorb the expansion of the outer package resulting from the increased internal pressure of the secondary battery so that the pack case does not expand or become deformed when pressed by the expanded outer package.
However, films of the laminated film outer package for accommodating the battery element are bonded by heat welding or another technique to enclose the battery element. While the film outer package repeatedly expands and contracts according to the internal pressure, the supporting point for deformation at the expansion and contraction is the bonded part. Accordingly, bending stress would constantly be concentrated on the bonded part. In the film bonded part, especially, the stress is likely to be concentrated on a discontinuous portion having a stepped structure or the like, e.g., a portion through which an electrode terminal protrudes outside, an explosion-proof working part having lower bonding strength than the bonded part except for such working part and serving as a safety valve in case of excessive internal pressure, and the like.
In Patent Documents 1 and 2 mentioned above, the film outer package facing the openings or the space formation part could freely expand and contract. However, there is no disclosure about the position of the openings or space formation part in the pack case. If these openings or space formation part exist in the bonded part of the film outer package, the stress resulting from the expansion and contraction will repeatedly be exerted on the bonded part. If the openings or space formation part exist in the bonded part having discontinuity for the electrode terminal, the explosion proof working part, or the like, the stress concentration caused by the expansion and contraction would be further increased. This may cause fatigue failure in the bonded part of the laminated film.
In aforementioned Patent Document 3, the retainers are placed in contact with the sides of the battery element inside the synthetic-resin film, so that damages to the battery element can be prevented. However, there is no disclosure about the relief of stress concentration on the bonded part of the synthetic-resin film resulting from the expansion and contraction of the film, and therefore the retainer does not seem to serve to relieve the stress. Thus, the stress when repeatedly applied may cause fatigue failure in the film.
The present invention has been made to solve at least one of the above problems and has a purpose to provide a laminate type secondary battery in which a battery element is accommodated in a sealed outer package made of laminated films, the secondary battery being capable of suppressing the concentration of stress on a bonded part of the laminated films even if expansion and contraction are repeated in response to the change of internal pressure, and to provide a battery pack thereof.